The invention relates to devices and methods for the introduction of ions into electrostatic ion traps (Kingdon ion traps), in whose internal DC field the ions can oscillate harmonically in a potential well in the longitudinal direction, essentially decoupled from their transverse motion. Mass spectrometers can only ever determine the ratio of the ion mass to the charge of the ion. In the following, the term “mass of an ion” or “ion mass” always refers to the ratio of the mass m to the number z of positive or negative elementary charges of the ion, i.e. the elementary charge related mass m/z (or charge-related mass, for short). Various criteria determine the quality of a mass spectrometer, the main ones being the mass resolution and the mass accuracy. The mass resolution is defined as R=(m/z)/Δ(m/z)=m/Δm, where R is the resolving power, m the mass of an ion, measured in units of the mass scale, and Δm the width of the mass signal at half maximum, measured in the same units.
Kingdon ion traps are electrostatic ion traps in which ions can orbit around one or more inner electrodes or oscillate between several inner electrodes. An outer, enclosing housing is at a DC potential, which ions with a predetermined sum of kinetic and potential energy cannot reach. In particular, it is possible to use Kingdon ion traps as electrostatic ion guides. To this end, an inner wire electrode must be surrounded by an outer electrode in the form of an enveloping tube with open ends. These Kingdon ion guides may compete with the better known RF multipole ion guides, which are based on the effect of pseudopotentials generated by RF voltages. In contrast to these, Kingdon ion guides convey ions of all masses in the same way, i.e. they do not have lower or upper limits for ion masses. Kingdon ion guides should be operated under ultrahigh vacuum in order not to lose any ions, while RF ion guides are usually operated at vacuum pressures in the range of 10−2 to 10 pascal.
In special Kingdon ion traps, which are particularly suitable as mass spectrometers, the outer electrodes have the form of almost closed housing electrodes; the inner surfaces of the housing electrodes and the outer surfaces of the inner electrodes are designed so that, firstly, the motions of the ions in the longitudinal direction of the Kingdon ion trap are decoupled from their motions in the transverse direction as completely as possible and, secondly, a parabolic potential well in the longitudinal direction is generated in which the ions can oscillate harmonically. The oscillation frequency depends on the charge-related mass m/z of the ions.
Kingdon ion traps can be designed in which the ions can swing transversely in a center plane between one or more pairs of inner electrodes, as described in detail in document U.S. Pat. No. 7,994,473 B2 (C. Köster; GB 2448413 B; DE 10 2007 024 858 B4). In the text below, these ion traps will be called “Kingdon swing ion traps”, or “swing traps” for short. Kingdon ion traps of the Orbitrap® type (Thermo-Fisher Scientific), as disclosed in the patent specification U.S. Pat. No. 5,886,346 (A. Makarov), can also be used, however; they are referred to as “Kingdon orbit ion traps”, or “orbit traps” for short, in this description because the ions orbit around the single inner electrode.
In the present document, the term “Kingdon ion trap” refers only to these special types in which ions can oscillate harmonically in the longitudinal direction, essentially decoupled from their motions in transverse direction. They can be used as mass spectrometers by measuring the harmonic oscillations of the ions in the longitudinal direction with the aid of the image currents influenced by the ion movements in suitable electrodes, for example in split housing electrodes. From the image current transients, the oscillation frequencies of the ions are determined by Fourier transformation. From the ion oscillation frequencies, the masses can be calculated. Thus these types of mass spectrometer, as also ion cyclotron resonance mass spectrometers (ICR-MS), belong to the general group of Fourier transform mass spectrometers (FTMS).
In the document US-2010-0301204-A1 (C. Koester and J. Franzen; GB 2470259 A; DE 10 2009 020 886 A1) which is incorporated herein by reference in its entirety, Kingdon ion traps of this category are described in detail and, in particular, devices and methods for the introduction of ions in such Kingdon ion traps are also disclosed. The advantages of the Kingdon ion traps, which particularly consist in a very high mass resolution, are also described. However, the high mass resolution can only be achieved if the operating voltage between inner and housing electrodes can be kept extremely stable, accurate to better than 10−6 over measuring times of at least several seconds. This voltage stability can best be maintained when the voltage does not have to be switched or changed in any way during operation.
For all Kingdon ion traps it is advantageous to inject the ions in the longitudinal direction at a location outside the potential minimum. The injected ions then immediately start to swing not only in the transverse x-y direction, but to oscillate also in the longitudinal direction z, without specially having to be excited to these oscillations. The z-position of the injection location of the ions determines the reversal points of the longitudinal oscillations. Thus, a special voltage generator is not required for the excitation of these oscillations in the longitudinal direction, i.e. no generator for “chirp” or “synch pulses”, as is required for the excitation of the ions in ICR mass spectrometers.
The method of introducing ions into the Kingdon ion trap according to the cited document US-2010-0301204-A1 consists in equipping the Kingdon ion traps with an electrically insulated entrance tube, which completely surrounds the ions during their introduction and guides them through the housing. This means that the ions can be introduced with a kinetic energy and at a potential which does not allow them to reach the housing electrodes of the Kingdon ion trap during their motions after they have been introduced. The only point of the housing which they can reach again after executing a number of oscillations is the introduction tube, as long as it is still switched to the potential for the introduction of the ions. A voltage generator can switch the entrance tube to different potentials. If the potential of the tube is switched back to a potential which roughly corresponds to the potential of the housing electrodes, the ions keep moving on their oscillation trajectories: they are trapped for ever until being released at will. The tube thus acts as an entrance gate which can be closed.
To introduce the ions, they are first collected outside the Kingdon ion trap, favorably in as small a cloud as possible, and then accelerated to form an ion beam which is decelerated before the Kingdon ion trap, and injected with reduced kinetic energy, while the entrance gate is open. This process of ion transfer from the cloud to the Kingdon ion trap causes a mass dispersion, however: the light ions arrive in the ion trap earlier than the heavy ions. The ion introduction process therefore takes a certain time from the arrival of the lightest ions to the arrival of the heaviest ions of interest. The entrance gate must remain open for this period, i.e. the tube must remain at the introduction potential for ions.
There is a distinct danger that light ions will find their way back to the tube and be discharged at the walls of the tube before the heaviest ions have arrived. All ions move on the same trajectories, independently of their mass. As described in the cited document US-2010-0301204-A1, by selecting an advantageous ratio of a characteristic length to the diameter of the Kingdon ion trap (the “aspect ratio”), it is possible to ensure that the introduced ions can only return to the opening of the entrance tube after several longitudinal oscillation cycles, preferably after about three to ten oscillations. If they return from the intermediate longitudinal oscillations, their transverse position should be located in some distance from the entrance tube. The aspect ratio determines the ratio of the number of transverse oscillation cycles to a longitudinal oscillation cycle. With a favorable aspect ratio, it is possible to extend the time until the lightest ions return to the tube so that even the heaviest ions of the range of interest have entered the Kingdon ion trap.
The tube is also disadvantageous, however. In order to pass a tube with a sufficiently large inside diameter and sufficiently sturdy wall thickness through the housing electrodes, and to allow for the tube to be insulated, the hole in the housing wall must be quite large. This means that the disturbance of the field inside the Kingdon ion trap becomes quite large. Furthermore, it is difficult to send an ion beam with relatively low kinetic energy through a narrow tube so that few ions are lost in the tube by wall contacts. Therefore, there is still a need for a switchable entrance gate for a Kingdon ion trap with minimized ion losses and perturbing effects on the field distribution inside the Kingdon ion trap, and corresponding introduction methods.